"Surround sound" is a term used in audio engineering to refer to sound reproduction systems that use multiple channels and speakers to provide a listener with simulate placement of sound sources. Typically, the listener is positioned between multiple speakers. By playing a sound at different intensities through one or more of the speakers, the sound is positioned with respect to the listener. In this way the listener may be "surrounded" with sound sources to create a more interesting or realistic listening experience. The device that plays back recorded audio signals through the speakers to achieve the surround sound effect is called a surround sound decoder.
Surround sound decoders are a common device in commercial movie theaters since surround sound is especially well-suited as a movie audio effect. By using surround sound along with a theater's large projection screen, an audience can be more completely immersed in the movie experience. Speakers toward the from of the theater, sometimes behind the screen itself, can localize sound to the right, left or center of the screen so that, for example, a movie character's voice can appear to come from the approximate position of the character on the screen. Additional speakers to the sides of the theater can be used to create sounds that are intended to be at the extreme sides of the screen or off-screen. Other popular locations for surround sound speakers are at the back and ceiling of the theater.
Surround sound decoders are also becoming more common in the consumer retail market. With the growing popularity of home entertainment centers, many consumers are creating high quality theater-like image and audio centers within their homes. Surround sound audio capability is one of the advanced features installed in such entertainment centers.
As a result of the success of surround sound in the theater and home markets the need for accurate, efficient and cost effective surround sound devices is increasing. Further, the audio processing technology in both theater and home systems has become increasingly digital. This places a burden on the surround sound decoder to meet the high quality specifications of digital audio processing such as low noise and wide dynamic range. However, typical surround sound decoders use analog components which fail to achieve the fidelity of today's digital audio. Also, the use of analog components in surround sound decoders causes the decoders to be sensitive to thermal effects and to decode inaccurately in some cases. The attempts to produce analog circuits that compensate for analog deficiencies require using high precision components, or adding additional circuitry to the decoder design, resulting in a more expensive decoder.
FIG. 1A shows an arrangement of speakers in a typical 4 channel surround sound system 100. Speakers 102, 104 and 106 play left ("L"), center ("C") and right ("R") channels, respectively. Speakers 108 and 112 are two speakers used to play the surround channel ("S"). Listening area 110 is positioned approximately in the middle of the speakers. In a cinema application, the movie screen (not shown) would be adjacent to, and parallel with, the left, center and right speakers.
The speakers are driven by channel signals generated by decoder 114. Decoder 114 derives the channel signals from a sound source signal generated by sound source 116. Sound source 116 may be a movie projector, compact disk ("CD"), laser disk, video tape, etc. A common format provides for the sound source to output two signals as left and right stereo signals to decoder 114. The decoder operates according to a surround sound format that specifies how the four L, R, C and S signals are encoded onto, and decoded from, the two stereo signals. One such format is promulgated by Dolby Laboratories and known as Dolby Pro Logic Surround. For a description of this surround sound format, and associated analog decoder, refer to papers published by Dolby Laboratories such as "Dolby Pro Logic Surround Decoder Principles of Operation," by Roger Dressier; 1988, 1993, reference no. S93/8624/9827.
FIG. 1B is a block diagram of a prior art analog surround sound decoder 120 such as that described in the reference above. Stereo inputs L.sub.in and R.sub.in at 122 are provided to the decoder from a signal source. According to the surround sound format, the stereo signals not only contain the standard left and right stereo signals, but are encoded with center and surround signal information. The center signal is encoded onto the SL and SR signals by dividing the center signal equally among L.sub.in and R.sub.in. In other words, the center signal is attenuated by 3 dB and added to each of the L.sub.in and R.sub.in signals. In contrast, the surround signal is phase encoded onto the stereo signals after first being band limited from 100 Hz to 7 kHz and processed with noise reduction encoding. The left stereo signal is added with the surround signal phase shifted by plus 90 degrees while the right stereo signal is added with the surround signal phase shifted by minus 90 degrees.
This encoding format provides for complete separation between the left and right signals. The surround signal is recovered by taking the difference between the left and right stereo signals. The center signal is recovered by adding the left and right stereo signals, thus cancelling the surround components which are 180 degrees out of phase.
Returning to FIG. 1B, the stereo signals L.sub.in and R.sub.in are decoded by passing the signals through bandpass filters 124 to remove strong low-frequency and high-frequency signals that may interfere with the decoding. Signals L and R are derived, as a result. A circuit at 126 sums L and R to generate the center signal, C. Circuit 128 subtracts L and R to generate the surround signal, R. Full wave rectifier circuits 130-136 are used to generate a direct current ("dc") voltage for each of the respective signals SL, SR, center and surround. These dc voltages are used to compute a log difference for each of the pairs SL/SR and center/surround by log-difference amplifier circuits 138 and 140.
The outputs of log-difference amplifier circuits 138 and 140 are dual polarity signals. The SL/SR dual polarity signal indicates that the left channel is dominant if the signal is, for example, positive. When the signal is negative it indicates that the right channel is dominant. The detection of a dominant signal is important to surround sound processing because of a human listener's tendency to focus on a single dominant sound. For this reason, the placement of sounds in the surround sound decoding and playback is heavily dependent on a dominant sound in the sound track. If a dominant sound is to come from the left, the quality and intensity of the right, center and surround channel outputs may be sacrificed to provide for a clear left originating sound.
Similar to the SL/SR dual polarity signal, the full wave rectifier circuits 134 and 136 and the log-difference amplifier circuit 140 output a center/surround dual polarity signal that indicates, in one polarity, that the center channel dominates and, in the other polarity, that the surround channel dominates.
The amplitudes of both of the SL/SR and center/surround dual polarity signals indicate to what degree the dominant signal of the pair dominates. A feature of the surround sound circuit is that it operates differently depending on the relative dominance of signals. If the dominance of a dual polarity signal exceeds a threshold level then the surround sound circuit switches from a slow mode to a fast mode of operation. This switching is performed in the dual time constant circuits 144 and 146 under the control of threshold switch circuit 142. The dual time constant circuits
Polarity splitter circuits 148 and 150 resolve the dual polarity dominance signals into four dominance control signals, or voltages, E.sub.L, E.sub.R, E.sub.C and E.sub.S. These control voltages are used to control eight voltage controlled amplifiers ("VCAs") 152, to amplify or attenuate the SL and SR signals. Thus, VCAs 152 output eight sub-term signals corresponding to SL and SR amplified according to each of the four control voltages.
The eight sub-term signals, along with the original SL and SR signals, are provided as inputs to combining network 154. Combining network 154 derives the left, right, center and surround signals from SL and SR and adjusts the strength of each of the signals according to the 8 sub-term signals. The surround signal is passed through filter 156, time delay 158, low pass filter 160 and noise reduction circuitry 162 before being output as the S channel signal. The left, right, center and surround signals may be subjected to balance, gain or other adjustments before being output as the L, R, C and S channel signals.
Although the analog surround sound decoder circuit of FIGS. 1A and 1B is adequate to perform surround sound decoding, the circuit requires the use of matched and calibrated analog components to perform sensitive functions such as the decoding of SL+SR and SL-SR signals. Also, because of the dual and quad nature of signal paths whose relative outputs play a large role in determining the circuit's operation any differences in the functional parameters of components in the paths could cause errors in the decoding. The circuit is also susceptible to all of the shortcomings of analog circuitry used in audio applications such as the introduction of noise, different characteristics of the circuit due to environmental (especially thermal) effects, non-uniform frequency response, etc.
Thus, it is desirable to produce a digital surround sound decoder. However, the successful design of a digital surround sound decoder is difficult since the accuracy and speed needed to process an audio signal at high resolution requires a high data throughput and a large number of instructions per second in the digital signal processing ("DSP") program. Further, to be useful the digital surround sound decoder must be compatible with existing surround sound formats.